Ignitor for sponge cathodes



G. LEWIN IGNITOR FOR SPONGE CATHODES Jan. 28, 1958 Filed June 30, 1954 INVENTOR Gerhard Lewin.

ig 4m ATTORNEY United States Patent r 2,821,511 IGNITOR FOR SPONGE CATHODES Gerhard Lewin, West Orange, N. J., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a cor poration of Pennsylvania Application June 30, 1954, Serial No. 440,447

2 Claims. (Cl. 252-516) This invention relates to ignitors, such as used as the make-alive electrode of an ignitron, and is of the general character identified as resistive ignitors.

There are several types of resistive ignitors known, such as silicon carbide crystals, the so-called converted ignitor, hot pressed mixtures of boron, boron carbide, boron nitride and carbon. All of these ignitors have normally a life of several thousand hours when used with the point penetrating a mercury pool cathode. However, U. S. Patent 2,651,737 issued to D. Marshall on or about Sepitember 8, 1953 describes an ignitron in which the cathode comprises a porous metal body, such as sintered iron, impregnated with liquid mercury, and which is known as a sponge cathode. When it is attempted to use such conventional resistive ignitors in engaging contact with a mercury-filled metal sponge cathode, they are likely to last for only a few minutes.

It is believed that the ignitor has only a point contact with the sponge cathode and, therefore, runs much hotter and erodes much faster than when dipped in a mercury pool. The erosion is probably due to decomposition of the ignitor. My application entitled Ignitor, filed August 30, 1951, now abandoned Serial No. 244,455 describes a form of resistive ignitor which does not decompose when eroded aud, therefore, is capable of operating with a life similar to that exhibited by conventional ignitors employed with mercury pool cathodes. This novel ignitor comprises a combination of a stable refractory insulator, such as alumina (A1 0 with one or more stable refractory conductor materials such as boron carbide in roughly equal proportions. While such an ignitor is satisfactory for many purposes, it has been found to fluctuate in resistance to such a degree that high firing voltages are sometimes needed to initiate operation of an ignitron after it has been shut down long enough to reach room temperature. On this account the simplest and cheapest form of firing circuit, i. e., the anode firing circuit, cannot be satisfactorily used for sponge cathode ignitrons using the ignitor described in my above-mentioned application.

One object of my invention is to provide an ignitor suitable for use with sponge cathodes which shall exhibit less variation in electrical properties between its cold and continuously operating conditions than was possible in the prior art.

Another object is to provide an ignitor for sponge cathodes for which the operating voltage shall undergo less variation during life than ignitors of the prior art.

Another object is to provide an improved ignitor material for mercury arc and similar vapor-electric devices, particularly those of the sponge cathode type.

Another object is to provide a new and improved type of resistive conductor.

Still further objects of the invention will be apparent to persons skilled in the art to which it .appertains, as the description proceeds, both by direct recitation thereof and by implication from the context describing the draw- 2,821,511 Batented Jan. 28, 1958 ings in which the single figure is a view in mid-section of'an ignitron having an ignitor and a cathode in accordance with my invention.

Referring in detail to the drawings, an ignitron tube comprises a vacuum-tight enclosure 1 of metal enclosing a cathode 2 of sintered porous iron impregnated with liquid mercury. An anode 3 which may be of graphite is supported on a metal rod 4 which is connected to the top of tube 1 by sealing to a glass sleeve 5. A second glass sleeve 6 supports a metal rod 7 which passes downward, through a channel 8 in anode 3 and has a semiflexible lower end-portion connected to a resistive ignitor 9 embodying my invention which contacts cathode 2.

According to the invention here disclosed, the ignitor 9 comprises a mixture of one or more stable refractory insulator materials with one or more stable refractory conductor materials and with a third material which is less stable at elevated temperatures. The substances selected and amounts of each employed are selected with due consideration of the sputtering characteristics thereof so as to obtain erosion of all constituents at approximately the same rate. The presence of a third component which decomposes at elevated temperature eliminates the occasional need for a high firing voltage across the ignitor. In particular, I have found boron nitride to be a satisfactory third component to use.

To give a very specific example of an ignitor embodying the principles of my present invention, a unit may be made up from forty-two parts aluminum oxide (A1 0 thirty-six and one-half parts boron carbide and fourteen parts boron nitride. The addition to this combination of 7.5 parts titanium nitride, a good conductor, will still further reduce temperature variation of the ignitor resistance. All the foregoing figures are parts-byweight.

While I have described the foregoing ignitor 9 as particularly useful with sponge cathodes; it also has many desirable characteristics for use with pool-type mercury and other self-renewing cathodes.

The several ingredients are, before use, masses of small particles and for brevity will be referred to as powders. The conducting particles, especially of the poor conductor ingredient, must be numerous enough in the surface of the completed body to permit the rapid upward movement of the ignitor arc. Hence the particle size must not be too large. To obtain uniform mixtures, components comprising thirty-six and one-half parts boron carbide (average particle size 3 microns), forty-two parts aluminum oxide (screened through a 325 mesh sieve) and fourteen parts boron nitride (average particle size 1 micron) are thoroughly mixed by ball-milling. The mixture is then pressed at a pressure of 1 ton per square inch and is sintered to a temperature of 1550 C. for one minute.

While I have described boron nitride as the additive material above other refractory substances which are decomposed at temperatures in the neighborhood of 1500 C. may be used in its stead. Also for many purposes the aluminum oxide may be replaced in whole, or in part,

v by thallium oxide.

I claim as my invention:

1. An ignitor for a sponge cathode ignitron made of a sintered mixture of about forty-two parts of an insulative material selected from the group consisting of thallium oxide and aluminum oxide, about thirty-six and one-half parts boron carbide, about fourteen parts boron nitride and about seven and one-half parts titanium nitride.

2. An ignitor for a sponge cathode ignitron made of a sintered mixture of about forty-two parts of an insulative material selected from the group consisting of thallium oxide'and alun linurn oxide, about thirty-six and one-half 2,276,656 Johnson Mar. 17, 1942 parts boron carbide and about fourteen parts boron nitride. 2,329,085 Ridgway Sept. 7, 1943 2 2,388,090 Scott Oct. 30, 1945 References Cited in the file of this patent 2,445,296 Wejnarth July 13, 1948 UNITED STATES P ENT 5 2,579,366 X 1951 2,206,792 Stalhane July 2, 1940 FOREIGN PATENTS 2,229,093 Knowles et a1. Jan. 21, 1941 103 5 Great i i f 13 2,269,861 Rennie 13, 1942 573,550 Great Britain Nov. 26, 1945 

2. AN IGNITOR FOR A SPONG CATHODE IGNITRON MADE OF A SINTERED MIXTURE OF ABOUT FORTY-TWO PARTS OF AN INSULTAVIT MATERIAL SELECTED FROM THE GROUP CONSISTING OF THALLIUM OXIDE AND ALUMINUM OXIDE, ABOUT THIRTY-SIX AND ONE-HALF PARTS BORON CARBIDE AND ABOUT FOURTEEN PARTS BORON NITRIDE. 